等差数列题!记等差数列｛An}的前n项和为Sn,若S1=4,S4=20,则该数列的公差d=?3Q!
网友回答
s1=a1=4
s4=a1+a2+a3+a4
=a1+a1+d+a1+2d+a1+3d
=4*a1+6d
=4*4+6d
=20所以d=(20-4*4)/6=2/3
======以下答案可供参考======
供参考答案1:
S1=A1=4
s4=a1+a2+a3+a4
=4*a1+d+2d+3d=4a1+6d=20d=(20-4*4)/6=2/3
供参考答案2:
a1=S1=4
S4=20=a1+a2+a3+a4
则S4-S1=a2+a3+a4=16
a2+a4=2a3
则3a3=16
a3=16/3
a3-a1=2d=16/3-4=4/3
d=2/3供参考答案3:
S1=A1=4
s4=a1+a2+a3+a4
=4*a1+d+2d+3d=4a1+6d=20d=(20-4*4)/6=2/3